1. Field of the Invention
The present invention relates to image exposure devices, and more particularly to image exposure devices equipped with an exposure head for forming a desired pattern on a photosensitive material.
2. Description of the Related Art
There are known image exposure devices with an exposure head for forming a desired pattern on a photosensitive material. This kind of exposure head basically comprises a light source; a spatial light modulator in which a great number of pixel portions are arranged for independently modulating light beams emitted form the light source according to a control signal; and an imaging optics system for forming an image of the light modulated by the spatial light modulator on the photosensitive material. The fundamental construction is described, for example, in A. Ishikawa, “Shortening of Development and Application of Mass Production by Maskless Exposure,” Electronics Mounting Technology, Technology Investigation Board Inc., Vol 18, No. 6, 2002, pp. 74-79.
U.S. Patent Application Publication No. 20030214571 discloses another exposure head, which comprises a light source; a digital micro mirror device (hereinafter referred to as a DMD) as a spatial light modulator equipped with a great number of micro mirrors; and a micro lens array in which a great number of micro lenses are arranged for individually converging the light beams modulated by the micro mirrors. In such a construction employing a micro lens array, even if an image exposed on a photosensitive material is enlarged, the light beams from the pixel portions of the spatial light modulator are converged by the micro lenses of the micro lens array. Therefore, the size of a pixel (spot size of each light beam) of an exposed image on the photosensitive material is kept small and the sharpness of the image can be kept high. Note that the DMD employed as a spatial light modulator in the aforementioned construction is a mirror device in which a great number of micro mirrors with a reflecting surface whose angle is independently varied according to a control signal are arranged on a semiconductor substrate such as silicon, etc. The exposure head shown in the aforementioned Publication No. 20030214571 further includes a single aperture array arranged in the stage after the micro lens array. The aperture array has a great number of apertures for individually restricting the aforementioned light beams. With operation of the aperture array, each light beam is shaped so that the size of a pixel becomes a fixed size on the photosensitive material, and crosstalk between adjacent pixels can be prevented.
However, in image exposure devices using a spatial light modulator such as a DMD, when turning each pixel portion of the spatial light modulator ON/OFF, the problem of chattering will arise. That is, it takes a slight time for the ON state or OFF state of the pixel portion to become stable. Since chattering causes the optical path of a light beam modulated by each pixel portion to fluctuate and the position of an exposed point to change on the photosensitive material, the sharpness of an exposed image will be reduced. An adverse influence due to chattering becomes more serious as switching of the ON and OFF states of the pixel portion is enhanced for high-speed exposure.
Also, if stray light that originates in a spatial light modulator, or marginal light is produced and reaches a photosensitive material, it can be another cause of the sharpness of an exposed image being reduced in image exposure devices. As described in the aforementioned Publication No. 20030214571, if a single aperture array is arranged in the stage after the micro lens array, it is possible to remove the stray light and assure a high total extinction ratio (ratio of the light quantity obtained by all pixels in the ON state and the light quantity obtained by all pixels in the OFF state). However, if stray light is to be removed by only a single aperture array arranged in the stage after the micro lens array, the size of each aperture and the position of the aperture array must be very strictly determined according to the diameter of the imaging components of each light beam collected by the micro lens array, and consequently, alignment of the aperture array and other components becomes extremely difficult.